A keratometer and a corneal topographer are known as ophthalmologic apparatuses for observing and measuring a cornea surface and the shape thereof. Generally these types of ophthalmologic apparatuses project light of a desired pattern onto the cornea surface. Then the state of the cornea surface, shape of the cornea, or other characteristic is measured by observing, photographing and analyzing a reflection image of the light pattern from the cornea surface.
The cornea surface is normally covered with a lacrimal fluid to form a lubricous fluid layer (lacrimal layer) on the cornea surface. The lacrimal layer gradually becomes thinner during the time the eyelid remains open after blinking and is eventually eliminated. When one's gaze is fixed on a monitor for an extended period, the number of blinks is reduced and the lacrimal layer is eliminated (a state of dryness referred to as dry eye). Moreover, due to individual differences in the rate in which the lacrimal layer is eliminated, there are people whose lacrimal layer reaches a state of dryness in a short period even without fixing their gaze on a monitor. People who experience this type of sudden dry eye daily often feel as if they have a foreign body in the cornea or the eyelid and thus often consult an eye specialist.
When eye specialists perform examinations on people whose chief complaint is dry eye, it is important to measure the state of the lacrimal layer formed on the cornea surface and to observe any changes over time. By studying the changes in the lacrimal layer over time from when the eyelid opens, the process by which the lacrimal layer is eliminated can be known. Thus ophthalmologic apparatuses are disclosed for measuring the state of the lacrimal layer formed on the cornea surface (for example, Japanese Laid-open Patent Publication No. 2000-237135, and Japanese Laid-open Patent Publication No. 2004-321508).
The ophthalmologic apparatus described in Japanese Laid-open Patent Publication No. 2000-237135 comprises an optical projection system and an optical receiving system. This ophthalmologic apparatus comprises a slit lamp microscope and a CCD camera for observing an eye to be examined. In this ophthalmologic apparatus a fluorescent dye is dropped onto the eye to be examined and the CCD camera is used to photograph the gray scale of the fluorescent reflection from the cornea surface. Then the region of dryness is identified by comparing the strength of the photographed fluorescent reflection with a predetermined threshold value. Lastly, the area of this identified region of dryness is displayed on a monitor. This ophthalmologic apparatus can quantitatively evaluate the state of dryness of the lacrimal layer, however, the inspection is invasive, as a fluorescent dye must be dropped onto the cornea.
Note that paragraph (0033) of Japanese Laid-open Patent Publication No. 2000-237135 describes an item which can measure a dry eye state without dropping a fluorescent dye onto the eye to be examined. More specifically, the document describes an item which can detect changes in the degree of dryness of a cornea surface over time by observing an interference pattern, in an image photographed with a halogen lamp as the only illumination, created by reflections from both the top and bottom surfaces of the lacrimal layer. However, it is not easy to uniformly illuminate the entire cornea surface and observe the interference pattern created by the reflections from both the top and bottom surfaces of the lacrimal layer, and thus the degree of dryness for the cornea surface cannot be detected with great precision. As a result, a fluorescent dye is indispensable for accurately performing an examination of the state of a dry eye.
In the ophthalmologic apparatus described in Japanese Laid-open Patent Publication No. 2004-321508, a beam of light is projected as appropriate onto the eye to be examined, and a CCD camera is used to photograph the reflection image from the cornea surface and the retina. Then the state of the lacrimal layer is quantitatively analyzed by using changes, due to the degree of dryness in the lacrimal layer, in the shape of the cornea, and wavefront aberrations. Thus the state of the lacrimal layer can be inspected noninvasively with this ophthalmologic apparatus. However, this ophthalmologic apparatus has various constraints that effect the measurement since a reflection from the retina is used. For example, when examining a patient with a cataract, light is scattered in the eyeball (varying with the severity of the cataract) causing interference with the reflected light beam observed by the CCD camera. Moreover, in principle, the diameter of the pupil constrains the range of measurement.